A group of irrigators has voted in favour of pursuing High Court action over the Water Act, potentially threatening the Murray-Darling Basin Plan.

They argue the 2007 Act is unconstitutional and subsequent actions by state and federal governments are invalid, including drawing up the Basin draft plan.

The irrigators are part of the group Murray Valley United, which has about 500 members in South Australia, Victoria and New South Wales.

Their vote taken in the SA town of Barmera is a step toward taking a case to the High Court.

Group vice-chairman Danny Lee said the 2007 Water Act was unconstitutional because it undermined water rights of the states and irrigators.

"If we are right, the MDBA plan is finished, it's an illegal document," he said.

Mr Lee said irrigators were putting their livelihoods on the line by committing money to a legal fight.

"The implications of this Commonwealth Water Act and MDBA plan getting up are so horrendous to the communities of the Murray-Darling Basin that that cost is very minimal compared to the damage that will be caused if we are wrong," he said.

Irrigator Caren Martin said the issue needed to be settled.

"Every citizen of this country has the right to constitutional account by their governments," she said.

A second and final vote was later taken at Merbein in north-west Victoria and also backed taking legal action.

A previous case from the group was dismissed by the Federal Court in 2010.

Bickering

Nationals Senate leader Barnaby Joyce said the Federal Government must act swiftly, before the Murray-Darling Basin Plan was derailed by bickering parties.

Senator Joyce toured the Coorong region on Tuesday at the invitation of irrigators.

He condemned the Barmera vote in favour of a legal challenge and said such actions would only prolong the river system's state of malaise.

"When people talk about legal challenges ... we will have no outcome, there will be no plan for South Australia or for anywhere else," he said.

A legal expert said any High Court challenge by the irrigators was destined to fail.

Professor Jennifer McKay from the University of South Australia said the Water Act had been carefully drafted to avoid constitutional issues.